Chiltern tunnel with crosspassageway, via HS2

HS2 progression: Florence and Cecilia reach the halfway point of their Chiltern excavation journey

The ongoing 10-mile HS2 tunnel excavation journey under the Chilterns has officially reached the halfway point, seeing ‘Florence ‘and ‘Cecilia’, the two giant tunnel boring machines (TBM), clear the way over an 18-month excavation process.

These two gargantuan 2,000 tonne machines have excavated twin tunnels between the M25 and South Heath in Buckinghamshire, set to give environmental protection whilst improving the interconnectivity between London, Birmingham and the North of England.

Accompanying the tunnelling process, project engineers have also successfully excavated five shafts that will enable ventilation and potential emergency access near Chalfont St Peter, Chalfont St Giles, Amersham, Little Missenden and Chesham Road.

Throughout the boring process, there has been more than 1.3 million cubic metres of chalk and flint excavated, enough to fill more than 500 Olympic swimming pools. To maintain an environmentally sustainable construction process, these materials will be utilised to progress with an ambitious chalk grassland restoration project at the south portal of the tunnel, which will see the creation of 127 hectares of new landscaping, wildlife habitat and biodiverse chalk grassland. To pump these materials to the surface, they are first mixed with water from a slurry, turning it into mulch for easier transport.

Martyn Noak, HS2 Ltd.’s Head of Tunnel Engineering, said:

“It’s great to be able to celebrate the half-way point of this 10-mile-long tunnel.  The whole team has put in a fantastic effort over the last year-and-a-half, and I’d like to congratulate the crews of both Florence and Cecilia for passing this important tunnelling milestone and thank them for all their hard work.

“This tunnel will take HS2 underneath the Chiltern hills, safeguarding the woodlands and wildlife habits above ground as well as significantly reducing disruption to communities during construction and operation.  Once complete, HS2 will offer low carbon journey options linking London with the major cities of the north and releasing capacity for more freight and local trains on our existing mainlines.”

The TBM’s themselves operate as a self-contained underground factory, encompassing an entire process of tunnel digging, lining them with concrete wall segments and grouting them into place at a speed of around 15 metres per day. Due to the sheer size and scales of this machinery, a crew of up to 17 people are required to operate each TBM, working in segmented shift patterns to ensure it runs day and night. A further 100 workers are there to provide support on the surface, managing the logistics and maintaining the smooth progress of the tunnelling operation.

Accompanying the Chiltern tunnel project, HS2 has also launched two TBMs to begin the London tunnels, celebrating the inaugural breakthrough with the completion of the first of two tunnels under Long Itchington Wood in Warwickshire.

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